Printed circuit boards (PCBs) are the benchmark for mounting electronic circuit components in today's communications hardware. Conventional PCBs include a rigid substrate to provide support for mounting electronic components in communications devices. In addition, conductive materials are plated over such substrates and etched to provide electrically conductive traces for interconnecting these components. For many communication devices, an area or a whole layer of such a PCB is often reserved as a ground plane serving as a reference ground for mounted electronic circuitry. Therefore, PCBs may be manufactured with multiple layers, each interconnected with conductive vias, to further provide electrical connections for complex electronic circuitry.
For many communications devices, antennas are typically formed on the same PCBs, which also carry transmitting and receiving radio frequency (RF) circuitry. A common technique employed to form antennas on PCBs is to simply etch an antenna trace, similar to the traces mentioned above, having an antenna feeder trace coupled to desired components on the PCB. Since space is limited in the ever-decreasing size of today's communications devices, such antenna traces are typically formed near one or more ground planes formed on the same PCB. In such arrangements, a portion of the PCB substrate, typically the area of a PCB having the highest density of electromagnetic energy, remains in between the antenna and the ground plane, leading to antenna efficiency degradation.
More specifically, as radio signals travel along an antenna trace, a portion of the signals are typically “lost” through energy loss or dissipation in the medium around the antenna trace, especially the medium between the antenna trace and the ground plane. The portion of total initial RF signals radiated into the surrounding space determines the antenna transmission efficiency (measured in dB) of the antenna. The same principle applies for antenna reception. Ideally, a 100% (0.0 dB) efficiency would be achieved if all of the RF signals traveling through the antenna were radiated into the surrounding space. However, as may be expected, the material from which a PCB is constructed has a large impact on the percentage of RF signals that are dissipated into PCB material surrounding the antenna structure. So-called “lossy” PCBs, such as the popular FR-4 PCB, are composed of materials (e.g., fiberglass and epoxy) that dissipate a relatively large amount of the signal. However, because lossy PCBs are both inexpensive to manufacture and process, manufacturers are eager to utilize them in an effort to drive down overall manufacturing costs. On the other hand, since RF signal loss becomes more important as transmission frequencies increase, the current trend in communications devices from 2.4 Ghz to 5 GHz technology may severely limit all future use of less expensive lossy PCBs.
Faced with the problem of RF signal dissipation, some manufacturers have chosen to employ low-loss PCBs, manufactured from materials that allow relatively low signal dissipation, in their communications devices. Low-loss substrates such as these usually have a dielectric loss coefficient (tg(d)) of about 0.01 or less. Examples of such substrates are the Rogers 4000 series, the PTFE, and the GTEK, each composed of special mixtures of materials such as fiberglass, epoxy, Teflon, ceramic, etc. Conventional antenna structures having an antenna trace formed on low-loss substrates usually have an antenna efficiency of about −0.5 dB or better, which translates into a radiation efficiency of about 90% or more. The same antenna structure on a lossy PCB, such as FR-4, usually has an antenna efficiency of about −2.0 dB, which translates into a radiation efficiency of about 65%. However, although providing increased antenna efficiency, low-loss PCBs tend to drive up overall product costs.
Another approach to reducing signal dissipation, and thus increasing antenna efficiency, has been to mount antennas above the substrate of the PCB. Those who are skilled in the art understand that air (e.g., an open space) between the antenna and the ground plane provides an optimum medium for antenna efficiency. The presence of almost any material in its place leads to decrease in antenna efficiency. Unfortunately, manufacturing such 3-dimensional antennas on PCBs, even low cost lossy PCBs, typically requires at least some human intervention during the manufacturing process. Of course, human intervention into the manufacturing process typically drives up the overall manufacturing costs of communications devices. In addition, human error that may occur during manufacturing detracts from overall product quality and longevity. Materials beyond the etched plated conductors used for antenna traces may further increase overall costs. Moreover, since such raised antennas are typically held on the substrate by a limited number of points, usually only two, the chance for antenna breakage during product use is increased.
Accordingly, what is needed in the art is an antenna structure, for use with a PCB, that does not suffer the RF signal dissipation experienced on prior art PCBs.